In conventional electrophotographic processes, a light-electroconductive insulating layer is uniformly charged (charging stage), the layer is then exposed to light and charges in the exposed areas are dispersed to form an electrostatic latent image (exposure stage), further charged colored fine particles (toner) are deposited on the electrostatic latent image to thereby convert the latent image to a visible image (development stage), the thus-formed visible image is transferred onto a transfer material such as transfer paper (transfer stage), and the visible image is permanently fixed by an appropriate means such as heating or an application of pressure (fixing stage) as described in U.S. Pat. Nos. 2,221,776, 2,297,691 and 2,357,809. Further, after the toner image is transferred, toner grains left on a photo conductor are removed to clean the surface of the material (cleaning stage).
Toner grains fed to a developing apparatus in the development stage are consumed, and fresh toner grains are generally fed to the developing apparatus from a toner feeding device called a "hopper". To conduct stable development, a metal oxide such as silica can be added to the surface of the toner to impart sufficient fluidity to the toner from the hopper to the developing apparatus and, further, to rapidly charge the toner to a proper charged amount when the toner is stirred in the developing apparatus.
The cleaning of the toner left can be generally made by means of a cleaning blade. However, a load is applied to the cleaning blade during the course of the continuous duplication stage. This results in disadvantages such as reversing or breaking the cleaning blade, and fusing the toner left onto the surface of the photo conductor by the pressure of the cleaning blade or by the frictional heat arising between the surface of the material and the cleaning blade. Further, there is a possibility that the toner grains which cannot be removed by the cleaning blade are accumulated and a failure in cleaning occurs. Accordingly, methods have been proposed wherein cleaning aids such as the metal salts of fatty acids are added to the toner to improve cleaning.
When the metal salts of the fatty acids as the cleaning aid are added to the toner, the reversing of the blade and the filming of the toner on the photo conductor can be prevented. However, this disadvantageously results in the occurrence of filming of the metal salts of the fatty acids, and, particularly, when a binary developer system is used, the metal salts of the fatty acids accumulate in the developer during the course of continuous duplication and the fluidity of the developers is changed.
JP-A-60-186851 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") proposes that fine particles of acrylic polymers such as fine particles of a methyl acrylate-butyl acrylate copolymer are added to the toner to improve cleaning and charging stability. However, this method has problems in that only when fine particles of the acrylic polymers are added to the toner as mentioned above, the agglomeration of fine particles of the acrylic polymers and the adhesion thereof to the photo conductors are increased. As a result, the fluidity of the toner is lowered and filming of fine particles of the acrylic polymers on the photo conductors occurs.
JP-A-1-291258 proposes to add acrylic polymers having a particle size of not larger than 0.05 .mu.m such as fine particles of polymethyl methacrylate. When such fine particles have a particle size of not larger than 0.05 .mu.m, a failure in cleaning can be prevented from occurring. However, frictional resistance to the blade cannot be reduced. Hence, the addition of the fine particles does not have an effect of solving the problems with regard to the reversing and breakage of the cleaning blade.
Binder resins which are conventionally used include polystyrene, styrene copolymers such as styrenebutadiene copolymers and styrene-acrylic copolymers, polyethylene, ethylene copolymers such as ethylene-vinyl acetate copolymers, poly(meth)acrylic esters, polyester resins, epoxy resins and polyamide resins. When toners containing these resins are used, the reversing and breakage of the blade in the cleaning stage occurs although the reasons for these unfavorable occurrences are not known. Accordingly, it has been demanded to develop a developer composition which is effective in preventing the reversing and breakage of the blade from occurring.
When the polyester resins are used as the principal component of the binder resin, polyester resins having an OHV/AV (wherein AV is an acid value of the polyester resin and OHV is a hydroxyl value thereof) value of not lower than 1.2 are generally used. This is because toners obtained by using a polyester resin having an OHV/AV value of lower than 1.2 have a high fusing temperature and poor fluidity in comparison with toners obtained by using a polyester resin having an OHV/AV value of not lower than 1.2. Further, a large amount of a surface treating agent such as hydrophobic fine silica powder must be added to the toners to impart sufficient fluidity. Also, when the above-described fine polymer particles as the cleaning aid are added, fluidity is further lowered disadvantageously.